The SLA inbred miniature pig herd, bred and maintained by the Ungulate Unit for the Immunology Branch, NCI typically produces a small litter and exhibits a variety of reproductive problems not found in the standard-sized pig. A uterine condition, cystic endometrial hyperplasia (CEH), exists in the herd and is directly involved in reducing the reproductive efficiency of the herd. Sows that are not bred until after one year of age have a high incidence of CEH and are incapable of reproduction. Regression or reduction of the incidence of CEH would improve greatly the reproductive performance of the herd. Previous studies have showed the SLA inbred miniature pig has a serum progesterone level considerably lower than the standard-sized pig. The effects of long term administration of progesterone on CEH will be studied.